Session P20: Focus Session: Multiferroics and other Functional Materials

The ferroelectric to antiferroelectric transition in multiferroic BiFe$_{1-x}$Cr$_{x}$O$_{3}$ epitaxial films

With the renewed interest in multiferroics, intensive investigations on BiFeO$_{3}$ films have enhanced the understanding of the nature of the ferroelectricity and the weak parasitic ferromagnetism. In contrast, despite having similar structural and chemical properties as BiFeO$_{3}$, little is know about BiCrO$_{3}$, due to the difficulty of synthesizing single-phase material. We have grown high quality BiCrO$_{3}$ epitaxial films by pulsed laser deposition and revealed that they exhibit antiferroelectricity with an electric- field induced ferroelectric phase. This antiferroelectricity is consistent with the picture of the Bi lone pair inducing polarization in bismuth-based perovskites. Furthermore, we have grown BiFe$_{1-x}$Cr$_{x}$O$_{3}$ solid-solution epitaxial films from BiFeO$_{3}$ and BiCrO$_{3}$ targets and observed a ferroelectric to antiferroelectric transition with increasing the Cr content. The interplay between the structural and (anti) ferroelectric properties and the role of the epitaxial strain will be discussed.   

Broadband Characterization of Multiferroic Thin-Films

The electromagnetic response of ferroelectric and multiferroic thin films at microwave frequencies is important for a fundamental understanding of these materials, as well for potential applications in electronics and communications systems. We explore the high-frequency response (to 40 GHz) of dielectric thin-film samples using a distributed measurement technique that utilizes patterned transmission line devices. We combine these measurements with measurements of lumped-element capacitors at lower frequencies (100 Hz - 100 MHz) to obtain true broadband measurements (100 Hz - 40 GHz) of the complex permittivity of thin film samples as a function of temperature, and electric- or magnetic-field bias.   

Quantum Monte Carlo calculations of BiFeO3

Multiferroic Bismuth Ferrite (BiFeO3) exhibits both ferroelectricity and antiferromagnetism, possibly enabling a connection between the two effects in the same material. While its antiferromagnetic character is relatively well-understood, experimental measurements of the spontaneous polarization vary significantly over two orders of magnitude, from 0.06 C/m$^2$ to 1.50 C/m$^2$. We cary out accurate quantum Monte Carlo calculations to estimate the cohesion energy and the ferroelectric distortion well depth. We discuss the mechanisms proposed to understand the variations of polarization experimental data in the light of our quantum Monte Carlo results.   

Ab-initio investigation of ferroelectricity in asymmetrically layered magnetic perovskites

In an effort to combine magnetism and ferroelectricity in a single material we are motivated to explore creative routes to ferroelectricity that allow the coexistence of magnetism. In this talk we present results from an ab-initio study of a system of asymmetrically layered magnetic perovskite oxides in which the asymmetric layering should induce a ferroelectric polarization. We investigate this prediction in a model system of La(Al,Fe,Cr)O$_3$, and find that a large switchable ferroelectric polarization can indeed be obtained, although it does not originate from the asymmetric layering. We examine the forces driving polarization in this system, and propose two- and three-dimensional heteroepitaxy as a general route to stabilizing novel ferroelectrics and multiferroics.   

Bond polarization induced by magnetic order

A number of recent experimental breakthroughs have revived interests in the phenomena of coupling of magnetic and electric (dipolar) degrees of freedom in a class of materials known as ``multiferroics". Some noteworthy observations include the development of dipole moments accompanying the collinear-to- helical spin ordering and adiabatic control of dipole moments through sweeping of applied magnetic fields, which all unambiguously point to the strong coupling of electric and magnetic degrees of freedom in these compounds. A number of phenomenological and microscopic theories has been advanced to establish the connection between noncollinear spin order and ferroelectricity. In particular the work of Katsura, Nagaosa, and Balatsky proposed a microscopic theory for the interplay between non- collinear magnetic order and the dipolar polarization of the electronic wave function induced by it. The magnetic (M) ion is modeled by three degenerate $t_{2g}$ levels experiencing some external magnetic field (to guarantee magnetic order) and subject to spin-orbit coupling. Two such magnetic ions are bridged by an intermediate oxygen (O) atom which itself has no spin-orbit interaction. Solving the model Hamiltonian perturbatively in the M-O hybridization amplitude, KNB finds an electronic polarization orthogonal to the M-O-M axis in the ground states of one and two holes.   

Resonant soft x-ray scattering study of the multiferroicity in TbMn$_2$O$_5$

TbMn$_2$O$_5$ is one of the fascinating multiferroic compounds whose spontaneous polarization can be controlled by applying magnetic field. Neutron diffraction measurements reported that incommensurate-commensurate transition of antiferromagnetic ordering is related to the appearance of ferroelectricity. In order to investigate the relationship between magnetic ordering and ferroelectricity associated with electronic structures of the Mn 3$d$ states, we measured soft x-ray resonant magnetic scattering of the single crystalline TbMn$_2$O$_5$ with photon energies around Mn $L_{2,3}$ edge. We observed that antiferromagnetic ordering of TbMn$_2$O$_5$ with incommensurate propagation vectors ($\frac12$$\pm \delta_x$, 0, $\frac14 $+$\delta_z$) coexists with antiferromagnetic ordering with a commensurate propagation vector ($\frac12$, 0, $\frac14$) in the ferroelectric phase (22 K $<$ T $<$ 37 K). Comparing the temperature dependence of resonant x-ray scattering and the arguments based on symmetry considerations, we discuss the magnetic ordering which leads to the magneto-electric effect in TbMn$_2$O$_5$.   

Growth and Characterization of Low Loss BaM-BSTO Multilayer Films

Ferrite/ferroelectric multilayer films are attractive as electronic materials because of the unique possibility of the electric field tuning of magnetic properties and vice versa. Up to now, however, it has not been possible to produce such layered structures with low microwave magnetic loss. The present work demonstrates the realization of pulse laser deposited low loss barium ferrite (BaM) in a BaM - barium strontium titanate (BSTO) layered film. The structure, from top to bottom, consists of a gold layer (30 nm), a polycrystalline BSTO layer (0.5 $\mu $m), another gold layer (30 nm), and a $c$-axis oriented BaM (0.5 $\mu $m) layer on a sapphire substrate. X-ray diffraction shows all components. Hysteresis loop and ferromagnetic resonance data show the properties of a low-loss $c$-axis oriented uniaxial BaM component. Capacitance measurements indicate a somewhat low but electric field tunable dielectric constant of the BSTO component. Supported in part by the ARO-MURI and ARO-DARPA-Seedling programs.   

Ferroelectric switching induced magnetic anisotropy in Fe/BaTiO$_{3}$ bilayers

Ferromagnetic/ferroelectric heterostructures have recently attracted significantly interest due to their potential applications in multifunctional electronic devices. We have recently predicted a magnetoelectric effect at the Fe/BaTiO$_{3}$ interface induced by ferroelectric polarization reversal [1]. In this report, calculations are being carried out on the magnetic anisotropy of Fe/BaTiO$_{3}$ films. Preliminary results show that the ferroelectric switching of the BaTiO$_{3}$ has appreciable effect on the magnetic anisotropy of magnetic Fe films. This should be of interest in multiferroic device applications. [1] Chun-gang Duan, S. S. Jaswal, E. Y. Tsymbal, Phys. Rev. Lett. 97, 047201 (2006).   

Magneto-electric Coupling in Ferromagnetic Cobalt/Ferroelectric Copolymer Multi-layer Films

We report on the magnetoelectric coupling of a thin multi-layer film sandwich consisting of ferromagnetic Cobalt (10 nm)/ferroelectric polymer (PVDF-TrFE)/ferromagnetic Cobalt (10 nm ).The metallic ferromagnetic 1mm wide electrodes are deposited perpendicular to each other through a shadow mask. The ferroelectric polymer films (53nm thick) are deposited by the Langmuir-Blodgett technique. The ferromagnetic and ferroelectric layers of the samples have been characterized by the Magneto-Optical Kerr Effect (MOKE) and the pyroelectric effect, respectively. After electrical saturation, the sample is placed in a magnetic field perpendicular to the plane of the sample. A large magneto-electric coupling is observed, with the pyroelectric response decreasing by $\sim $ 30{\%} on application of a 2kG field. Our observations indicate that the polarization change occurs abruptly at the closing of the magnetic hysteresis loop, shows little hysteresis and is even with magnetic field. The change is far too large to be accounted for by the magnetostriction of Co. Possible explanations for this unexpectedly large effect are discussed. NSF Grant No MRSEC DMR-0213808   

Natural off-stoichiometry and asymmetry in $p$/$n$-dopability of wide-gap oxides$^{1}$

Oxides such as In$_{2}$O$_{3}$ and ZnO can be doped $n$-type and are naturally anion deficient, while oxides such as NiO and Cu$_{2}$O are$ p$-type and tend to be naturally metal-deficient. Furthermore, they exhibit the property of transparent conductivity, unlike most oxides. To decipher these phenomena, we perform thermodynamic simulations based on first-principles calculated formation energies of many neutral and charged defects. We find that the metal-vacancies (and not O-interstitials) in NiO and Cu$_{2}$O are responsible for their simultaneous metal deficiency and $p$-type conductivity. The O-deficiency of In$_{2}$O$_{3}$ and ZnO is caused by the O-vacancy V$_{O}$ (and not the metal interstitials). Since V$_{O}$ has a \textit{deep} level in the gap, it does not provide for equilibrium stable $n$-type conductivity. We suggest, however, that a metastable state of V$_{O}$ in In$_{2}$O$_{3}$ and ZnO can cause persistent photoconductivity, and can explain the paradoxical coexistence of coloration (deep absorption level in the optical range) and conductivity (shallow level), which is observed after metal rich growth. By calculating the band offsets, we further show that the $p$-type dopability of NiO is facilitated by the relative high energy of the valence band maximum, while the $n$-type dopability of ZnO is facilitated by the relative low energy of the conduction band minimum. \newline $^{1}$Funded by DOE-BES under contract DE-AC36-99GO10337   

Raman scattering studies of resistance-changing NiO films with and without IrO$_{2}$ buffer layers

NiO films are known to exhibit resistive memory switching behavior and inserting thin IrO$_{2}$ layers between electrodes and the NiO film is claimed to minimize the dispersion of memory switching parameters, thus greatly improving the device properties. We present Raman scattering results of a NiO film, a NiO film with a 20 nm-thick IrO$_{2}$ layer, and a NiO film with a 50 nm-thick IrO$_{2}$ layer. We discuss the microscopic structural changes in the three different films and their relations to the switching behavior changes. We also discuss the role of IrO$_{2}$ buffer layers in the device structures.   

Molecular Beam Epitaxy of MgO on Perovskite Substrates

Rock salt oxides are promising interface layer materials for the integration of multifunctional oxides with semiconductors (Si, SiC, and GaN). Although rock salt oxides were previously grown on a wide range of semiconductor (Si and GaAs) and oxide (LaAlO$_{3}$ and SrTiO$_{3})$ substrates, the influence of lattice mismatch on the crystalline quality of the films was not studied. MgO thin films were grown by molecular beam epitaxy on LSAT, LaAlO$_{3}$ and SrTiO$_{3}$ perovskite substrates to investigate the effects of lattice mismatch on the film crystal quality. Despite a lattice mismatch of $\sim $7.9{\%} and $\sim $9{\%}, respectively, epitaxial growth of MgO was achieved on SrTiO$_{3}$ and LSAT substrates. Films grown on LaAlO$_{3}$ substrates exhibiting a lattice mismatch of $\sim $10.5{\%} were polycrystalline, yet epitaxial MgO on LaAlO$_{3}$ was deposited after the introduction of a SrTiO$_{3}$ buffer layer. The effects of deposition rate, substrate temperature, ozone flux, SrTiO$_{3 }$buffer layer thickness and stoichiometry were also investigated. This work was supported by the Office of Naval Research under grants N00014-05-1-0238 and N00014-06-1-1018.